JavaScript: Function.prototype.apply

f.apply( obj)

evaluate f() with this Binding having value of obj.

f.apply( obj, arg_list)

elements of the array arg_list is passed to the function as arguments. arg_list is a array or Array-Like Object.

// example of using apply
function ff() { return this; }
const x2 = {kk:373};
console.log( ff.apply( x2 ) ); // { kk: 373 }

Example with 2 arguments:

// example of using apply

function ff(a,b) {
    this.x = a;
    this.y = b;
}

const x3 = {};

ff.apply(x3, [7,8]);

console.log( x3 ); // { x: 7, y: 8 }

// Second arg of apply can be Array-Like Object
function ff(a,b) { return [this, a, b] }
console.log( ff.apply( {}, {0:7,1:8,length:2}) );
// [ {}, 7, 8 ]

Reflect.apply

Tip: you might want to use Reflect.apply instead.